1. Field of the Invention
The invention concerns optical communications using wavelength division multiplexing (WDM).
2. Description of the Prior Art
In this type of communication, a plurality of information channels carried by different wavelengths are conveyed simultaneously in optical links. The invention is more particularly concerned with receiving and extracting one or more channels of a multiplex of this kind.
Channel extraction is effected in the routing nodes of an optical network, for example. In order to switch one of the channels of the multiplex received by the node to a particular optical link, that channel is extracted by filtering, for example using a demultiplexer or a tunable filter tuned to the wavelength of the channel, after which the wavelength of the carrier is modified using a wavelength converter to a value associated with the required link.
However, all optical filtering introduces power losses due to the filter itself and the filter input and output coupling means. A filter device therefore produces attenuation that generally has to be compensated by amplification. An optical amplifier is used for this, for example, having a gain at least equal to the attenuation coefficient of the filter, the attenuation coefficient being defined as the ratio (greater than unity) of the input optical power to the output optical power of the filter.
The drawback of such amplification is that it is always accompanied by a degraded signal to noise ratio of the optical signal obtained. This is due to the fact that an optical amplifier generates noise that is substantially proportional to its gain, known as "amplified spontaneous emission" or "ASE". To compensate the attenuation of an optical device with the signal to noise ratio degraded as little as possible, it is necessary to place the amplifier on the input side of the attenuator device. In this case the signal to noise ratio is not degraded by the attenuation, as would be the case if the amplifier were on the output side.
However, in the case of filtering, the solution that provides the best signal to noise ratio is not optimal from the point of view of use of the amplification means. By definition, filtering to extract some channels from a multiplex eliminates the other channels. As a result, the increase in power applied by the amplifier to the channels that are thereafter eliminated by filtering is of no utility.
An aim of the invention is therefore to propose a solution that optimizes the amplification means. For this, account is taken of the fact that it is not always essential for the signal to noise ratio of the amplified and filtered signal to be the best possible ratio. In practise, a limit value of the signal to noise ratio can be set that is sufficient for subsequent processing of the signal. Likewise, the optical power necessary for such processing can be evaluated.